There is known an amplifier circuit that has a first transistor which is connected to an input terminal and whose gate is grounded in an AC manner and a second transistor which is connected to the first transistor in series and whose gate is grounded in an AC manner (for example, see Patent Document 1). A first resonance circuit includes an inductor and a capacitance connected to the gate of the second transistor in series. The inductor and/or capacitance of the first resonance circuit is (are) variable.
Further, a bidirectional amplifier which amplifies an RF signal is known (for example, see Patent Document 2). A first input port is electrically coupled to an end of an RF transmission line. A second input port is electrically coupled to an opposite end of the RF transmission line. At least one variable matching network is electrically coupled to the transmission line between the first input port and the second input port. At least one amplifier device is electrically coupled to the RF transmission line between the first input port and the second input port. At least one amplifier device amplifies an RF signal applied to either one of the first input port or the second input port. The variable matching network carries out different impedance matching to the RF signal applied to the first input port and the RF signal applied to the second input port.
Further, there is known a high-frequency amplification device which has a signal distribution means which distributes a high frequency, a phase adjustment means which adjusts a phase of one of modulated waves having been distributed by the signal distribution means, and an amplification means whose input signal is the phase-adjusted modulated wave (for example, see Patent Document 3). An envelope signal detection means detects an envelope signal of the other modulated wave having been distributed by the signal distribution means. A control voltage generation means has an analog-digital conversion means, a digital-analog conversion means, and a storage means. The analog-digital conversion means converts an envelope signal detected by the envelope signal detection means into a digital signal. The digital-analog conversion means converts a digital signal into an analogy voltage. The storage means connects the analog-digital conversion means and the digital-analog conversion means. An amplification means is constituted with an amplifier circuit made of a cascade amplifying element in which a drain terminal of a grounded-source FET and a source electrode of a grounded-gate FET are directly connected with each other. By giving an output of the control voltage generation means to a gate of the grounded-gate FET to change bias distribution of the grounded-source FET and the grounded-gate FET, an output level of the amplifier circuit is controlled in accordance with an envelope signal.
[Patent Document 1] International Publication Pamphlet No. 2007/099622
[Patent Document 2] Japanese Laid-open Patent Publication No. 2004-7703
[Patent Document 3] Japanese Laid-open Patent Publication No. 9-18247
In a case where a plurality of transistors whose gates are grounded in an AC manner is connected in series, a high frequency input signal brings about positive feedback and oscillation since the grounded-gate transistor is a non-inverting amplifying element, and there is a problem that amplifying operation becomes unstable. Further, when the amplifying operation is intended to be stabilized by using resistance, a gain of amplification is reduced. It is difficult to attain both stabilization and a high gain in amplification.